Generally, automatic transmissions used in vehicles enable the transmitting of drive power of an engine to a gear train through a torque convertor, the torque convertor acting as a hydraulic clutch. The gear train is connected to one or more planetary gear set, and uses one of the three elements of a planetary gearset (sun gear, ring gear, and planetary carrier) to be an input element, one to be a reaction element, and the remaining element to be an output element such that an appropriate shift ratio can be output.
A plurality of friction elements and a one-way clutch are used to enable selection of the above elements of the planetary gearset as an input element, reaction element, and output element. Also, the friction elements are selectively controlled at proper times to realize shifting by a hydraulic control system, the hydraulic control system including solenoid valves which are controlled by a transmission control unit (TCU).
The hydraulic control system, as shown in FIG. 4, generally includes an oil pump 2 which continuously pumps oil to create hydraulic pressure while the engine is running, a torque convertor 4 for transmitting engine power to an input axle of the transmission, a regulator valve 6 which controls the hydraulic pressure created by the oil pump 2 to a fixed level of pressure (i.e., line pressure), a torque convertor control valve 8 for controlling the hydraulic pressure of the torque convertor 4 and that for lubrication to a fixed level, and a damper clutch control valve 10 for controlling hydraulic pressure operating on a damper clutch mounted in the torque convertor 4.
There is further provided a reducing valve 12 which reduces hydraulic pressure to a fixed level below line pressure, a manual valve 14 moving in unison with a shift select lever (not shown) and which converts lines to supply or exhaust line pressure to or from each valve, and a shift control valve 16 for converting lines by ON/OFF operations of two shift control solenoid valves S1 and S2. Also, the hydraulic control system includes an input control valve 18 controlled by an input control solenoid S3 and which prevents shift shock during shifting, and an N-R control valve 20 for preventing shift shock during shifting from a neutral N range to a reverse R range.
Further, there is provided a 1-2 shift valve 22 for controlling both the flow of line pressure when shifting from a first speed to a second speed and pressure lines connected to friction elements operating when reversing, and a 2-3/4-3 shift valve 24 operating by line pressure and which supplies operating release pressure to a second friction element C2 and operating pressure to a third friction element C3. Also included are an end clutch valve 26 supplying operating pressure to a fourth friction element C4, and a fifth friction element C5 operating by directly receiving hydraulic pressure from the manual valve 14 in the reverse R range. And finally, there is provided a rear clutch release valve 28 for exhausting operational pressure of a first friction element C1 when shifting from a third speed to a fourth speed and controlling the amount of time pressure is supplied to the first friction element C1 when shifting from the fourth speed to the third speed to prevent shift shock.
Referring to FIG. 3, the manual valve 14 used in the hydraulic control system structured as in the above includes a valve body provided with a plurality of ports, and a valve spool 100 moving in unison with the shift select lever inside the valve body.
The valve body comprises a first port 102 communicating with the oil pump 2, second and third ports 104 and 106 communicating with the regulator valve 6 through a drive pressure line and a reverse pressure line, a fourth port 108 which supplies line pressure supplied to the first port 102 to the shift control valve 16 and the input control valve 18 in a drive D range, a fifth port 110 for supplying reverse pressure in the reverse R range, and exhaust ports EX1 and EX2 provided on one end of the valve body.
The valve spool 100 includes a first land 112 for selectively communicating the first and second exhaust ports EX1 and EX2 to the third and fifth ports 106 and 110; a second land 114 for selectively communicating the first port 102 to the third and fifth ports 106 and 110, in the reverse R range, the second port 104 to the first port 102 in the neutral N range, and the second port 104 with the fourth port 108 in the D, 2, and L ranges; and a third land 116 for communicating the first port 102 with the second port 104 in the parking P range. Also, the valve spool 100 includes a connecting portion 118, formed on a side opposite that to which the first land 112 is formed, which is connected to the shift select lever. Further, an exhaust passage 120 is formed along an inside the valve spool 100 from the connecting portion 118 to the second land 114 where an orifice 122 is formed.
When the above valve spool 100 is moved, as shown with the phantom line, to the neutral N range, the hydraulic pressure supplied to the first port 102 controls the regulator valve 6 via the second port 104. However, when the valve spool 100 is moved to the parking P range, lubricating fluid is unable to be supplied to the torque convertor 4.
That is, as shown in FIG. 3, when the valve spool 100 is positioned at the parking P range, as the orifice 122 communicates with the first port 102 and one end of the exhaust passage 120 is in a completely open state, the line pressure supplied from the oil pump 2 passes through the exhaust passage 120 and is completely exhausted therethrough. As a result, pressure is not supplied to the second port 104, even if the same is open, nor is pressure supplied to any other valve.
Accordingly, because lubricating fluid is unable to be supplied to the torque convertor when in the parking P range, if the engine is left to idle for a long period of time, it is possible for the torque convertor and other internal parts of the transmission to become damaged.